1. Field of the Invention
The present invention relates to voltage divider circuits, and more particularly to capacitor type voltage divider circuits having a low power-consumption.
2. Description of the Prior Art
FIG. 1 is the circuit diagram of a typical analog-to-digital converter of a flash type. This converter is the circuit for generating reference voltage signals and converting them into digital data, which comprises a reference voltage generator 100, a comparator 101 and an encoder 102.
In FIG. 1, several reference voltage signals Vr.sub.1 to Vrn-1 are generated by the reference voltage generator 100 and are provided respectively to inverting terminals (-) of a plurality of operational amplifiers in the comparator circuit 101. Then, a voltage Vin from an external input terminal is commonly applied to respective non-inverting terminals (+) of the amplifiers. The comparator circuit 101 compares each of the reference voltage signals with the external voltage, and resultant signals are provided to an encoder 102. As a result, the encoder 102 encodes the resultant signals and then outputs digital data DO to Dn which indicate whether the external voltage is at any level of the reference voltage signals.
In general, when several reference voltage signals are required in an integrated circuit such for an analog-to-digital converter and the like, it is difficult to separately generate and apply these reference voltage signals directly to the integrated circuit. For this reason, a voltage divider with a plurality of resistors has been used with such an integrated circuits, in which the plurality of resistors are serially connected between two reference voltage terminals and several reference voltage signals are supplied from respective connection between two adjacent resistors. This reference voltage divider circuit of a resistor type is shown in FIG. 2.
The divider circuit 100a of FIG. 2 is an example of the reference voltage generator 100 of FIG. 1. As shown in FIG. 2, several resistors R1 to Rn are serially connected between two source voltage terminals. From respective connection between two adjacent resistors, voltages Vr1 to Vrn-1 are applied and used as reference voltages of the comparator 101 of FIG. 1. For example, the reference voltage Vr1 can be given by the following equation: ##EQU1## where, R1 to Rn are resistance values of the serially connected resistors of FIG. 2, respectively, and Vr is a voltage from one of the source voltage terminals.
By this resistor type voltage divider, several reference voltage signals can be obtained from respective connections of the voltage divider 100a, as given in the equation (1).
However, this resistor type voltage divider 100a has a simple structure, but has two problems as described below.
One of the problems is that it is difficult to obtain precise reference voltage signals. This is because a relative error of about 2% exists between adjacent resistors. Thus, in a case when such a resistor type voltage divider is incorporated in a flash type analog-to-digital converter, there arises a problem that it is difficult to obtain resolution of 8 bits and more.
The other of the problems is that current flows continuously in an analog-to-digital converter (hereinafter, referred as to "A/D converter") regardless of using reference voltage signals. This is because the divider has several resistors between two source voltage terminals. As a result, there arises a problem of large power-consumption. If resistors having high resistance are embodied in the voltage divider, the power-consumption can be relatively reduced. However, since such resistors do not satisfy the condition that an internal resistance must to be low, operation speed is seriously reduced in the case when the voltage divider having the resistors is connected to peripheral circuits. Also, in the case when resistors having a high resistance are embodied in an integrated circuit, such an integrated circuit is substantially increased in size.
To overcome the above-described problems, a capacitor type voltage divider circuit as shown in FIG. 3 has been proposed in Japanese Patent Publication No. SHO 58-48525. In this voltage divider, a technique for reducing reference voltage error is disclosed. Reference voltage error is generated due to error of a capacitor itself caused by processing variation.
In FIG. 3, capacitors C1 and C2 have the same capacitance. The capacitor C1 is connected between an output terminal Vout and a first switch S1. The capacitor C2 is connected between the output terminal Vout and a second switch S2. Each selecting terminal of the switches S1 and S2 is connected to one of first and second contact terminals V1 and V2 and switched by a clock signal CLK as shown in FIG. 4. For example, when the clock signal CLK is high level, i.e. during a time period T1, the selecting terminal of the first switch S1 is connected to the first contact terminal V1 and the second switch S2 is connected to the second contact terminal V2. Then, the output signal Vout1 of the voltage divider 100b is obtained by following equation. ##EQU2##
On the other hand, when the clock signal CLK is low level, i.e. during a time period T2, the first and second switches S1 and S2 are connected to the second and first contact terminals V2 and V1, respectively. Then, the output signal Vout2 of the voltage divider 100b is obtained by following equation: ##EQU3##
In this equation (3), when it is assumed that there is an error .DELTA.CO between the capacitors C1 and C2, C1=CO+.DELTA.CO, and C2=CO-.DELTA.CO, a voltage difference of 2.DELTA.V from the output terminal Vout exists as a ripple component, as shown in FIG. 4.
The ripple component can be eliminated by a low pass filter connected to the output terminal. However, since an integrator is embodied in each of the operational amplifiers of the comparator 101, the output signal Containing the ripple component is averaged in the respective operational amplifier. In FIG. 4, a one dot dash line indicates an averaged level of the output signal Vout of the voltage divider 100b.
Thus, in the capacitor type voltage divider, an error of the voltage level caused by a capacitance difference between capacitors can be reduced. However, there is a problem that noise caused by the switching operation of the switch can not be eliminated. Since the voltage divider also does not have a function for generated the reference voltage signals, there arises the problem that each level of the reference voltage signals is not constant, i.e is varied.